The present invention relates to mechanically strong, water-disintegrable agglomerates for use as a soil conditioner produced from dust particles emitted from kilns during the manufacture of cement. The present invention also relates to a process for forming such agglomerates.
The manufacture of cement is accompanied by the generation of large quantities of dust entrained in the hot stack gases vented from cement kilns. In order to prevent air pollution, a variety of dust collecting apparatus (e.g., bag houses, electrostatic precipitators) are used to collect cement kiln dust from the gaseous effluents of the kiln. Preferably, the collected cement kiln dust is recycled to the cement kiln as part of the raw material feed. However, cement kiln dust often contains unacceptably high levels of alkali (e.g., sodium and potassium) which make it unsuitable for return to the cement-making process. Alkali present in cement kiln dust recycled to the kiln eventually finds its way into the clinker product. High alkali content in the clinker has an adverse effect on the rate at which concrete made from the clinker gains strength and may also lead to slow, insidious deterioration of the concrete due to the reaction between the alkali and amorphous silica contained in the aggregate.
If the raw materials fed to the kiln are high in alkali content, some of the alkali must be removed from the cement making process in order to avoid the problems associated with elevated alkali levels in the clinker. Since alkalies present in the kiln feed tend to accumulate in the dust entrained in the kiln stack gases, discarding the collected dust is one way to control the alkali content of the clinker. The discarded dust is often disposed of by simply piling it adjacent the cement plant facility or hauling it to an abandoned quarry or other landfill. These disposal methods often lead to pollution of surface and ground waters as the water-soluble alkalies present in the dust are leached out by water percolating through the dust piles. In order to prevent pollution problems, cement manufacturers are required to invest considerable resources in cleaning up runoff from cement kiln dust storage before releasing it to the environment.
Over the years, several uses for discarded cement kiln dust directed at reclaiming this sizeable waste stream and avoiding the costs associated with dust disposal have been proposed and implemented with varying degrees of success. Agricultural use of cement kiln dust presents one of the most promising ways to convert discarded cement kiln dust into a valuable by-product of cement manufacture. See T. A. Davis, et al., "Disposal and Utilization of Waste Kiln Dust From Cement Industry", EPA Report No. 670/2-75-043 (May 1975). Two properties of cement kiln dust make it useful for application to cultivated soils: (1) an acid neutralization capacity which is about 80% of that of slaked or hydrated lime (Ca(OH).sub.2) and comparable to that of pulverized limestone (CaCO.sub.3); and (2) high concentrations of potassium, calcium and sulfur which are important soil nutrients. In the agricultural industry, large amounts of hydrated lime, limestone, dolomitic limestone (CaOMgO.multidot.2CO.sub.2) and quicklime (CaO) are used as liming agents to reduce soil acidity and provide a source of calcium. Agricultural uses alone could consume almost all of the cement kiln dust presently being discarded.
One serious hindrance to expanded agricultural use of cement kiln dust are the problems associated with its storage, handling and application to soils. Raw cement kiln dust is extremely fine, typically consisting of particles having an average particle size much less than about 100 .mu.m, with a large portion of particles often having a particle size less than about 6 .mu.m. As a result, the dust is easily carried away by the wind and is difficult to bulk-blend with other materials in preparing various fertilizer formulations. Furthermore, modern fertilizer application equipment for broadcast or row placement of solid fertilizers, is designed for handling free-flowing, granular or pelletized materials having an appreciable mean diameter, not dust. Consequently, numerous problems are encountered when such equipment is employed for field application of finely-divided cement kiln dust.
It has been proposed that one way to overcome these shortcomings would be to develop a process for granulating or agglomerating cement kiln dust to produce a mechanically strong pellet. See Soviet Patent 220277.